Paint coatings are surface protective coatings applied to substrates and dried to form continuous films for decorative purposes as well as to protect the substrate. Consumer paint coatings are air-drying aqueous coatings applied primarily to architectural interior or exterior surfaces, where the coatings are sufficiently fluid to flow out, form a continuous paint film, and dry at ambient temperatures to protect the substrate surface. A paint coating ordinarily comprises an organic polymeric binder, pigments, and various paint additives. In dried paint films, the polymeric binder functions as a binder for the pigments and provides adhesion of the dried paint film to the substrate. The pigments may be organic or inorganic and functionally contribute to opacity and color in addition to durability and hardness, although some paint coatings contain little or no opacifying pigments and are described as clear coatings. The manufacture of paint coatings involves the preparation of a polymeric binder, mixing of component materials, grinding of pigments in a dispersant medium, and thinning to commercial standards.
Latex paints for the consumer market ordinarily are based on polymeric binders prepared by emulsion polymerization of ethylenic monomers. A typical consumer latex paint binder contains a vinyl acetate copolymer consisting of polymerized vinyl acetate (80%) and butyl acrylate (20%). The hardness of the latex polymer must be balanced to permit drying and film formation at low application temperatures, which requires soft polymer units, while at the same time the polymer must be hard enough in the final film to provide resistance properties which requires hard polymer units. This is conventionally accomplished by designing a latex polymer with a moderately elevated Tg (glass transition temperature) but then lowering the Tg temporarily with a volatile coalescing solvent. Coalescing solvents function to externally and temporarily plasticize the latex polymer for time sufficient to develop film formation, but then diffuse out of the coalesced film after film formation, which permits film formation and subsequent development of the desired film hardness by the volatilization of the coalescent. Internal plasticization is based on coreaction of soft monomers with hard monomers to form a polymeric copolymer binder, such as 80/20 vinyl acetate/butyl acrylate, to obtain the desired film forming characteristics. If a lower Tg copolymer is used without a coalescing solvent, higher levels of soft comonomer are required to achieve a lower Tg, but then the final dried film would be undesirably soft, excessively tacky, readily stain, and readily pick up dirt.
A significant source of residual odor in latex consumer paints is directly due to the coalescing solvent. Coalescing solvents are typically linear (or slightly branched) glycol ethers and esters of 7 to 12 carbon atoms in length, which have boiling points typically above 200.degree. C., and solubility parameters appropriate for the latex of interest. One typical coalescing solvent ordinarily contained in commercial latex paints is 2,2,4-trimethylpentanediol monoisobutyrate (Texanol.RTM. Eastman Chemical Co.). The odor associated with the gradual volatilization of this solvent is considered objectionable by consumers. Quite often the odor lingers for days or weeks after the paint is applied and dried. All useful coalescing solvents are volatile and have similar objectionable characteristics. An additional deficiency in conventional exterior latex paints is the decline in crack resistance of the dried paint film approximately proportional to the evaporation of the coalescing solvent. While better coalescing solvents have a retention time of about one year in dried paint films, cracking starts to progressively appear after one year in dried paint films. Hence, the elimination of coalescing solvents, attendant objectionable odors, air pollution caused by volatile organic compounds (VOC), and film cracking deficiencies all represent a technical and marketing advance in the state of the art of consumer latex paints.
In polymer technologies unrelated to air-dry vinyl acetate latex paints, preformed polymers have been dispersed into monomers and emulsified in water, whereupon the monomers are then polymerized, such as disclosed in U.S. Pat. No. 4,373,054 pertaining to cathodic electrocoating, or in U.S. Pat. No. 4,313,073 pertaining to alkyd prepolymers; or U.S. Pat. No. 4,588,757 pertaining to laminating adhesives, or in U.S. Pat. Nos. 3,953,386 and 4,011,388 pertaining to aqueous emulsion blends containing cellulosic ester/acrylic polymers.
In said commonly assigned Ser. No. 019,633 filed Feb. 18, 1993, certain non-volatile softening oligomeric modifiers were found compatible with a polyvinyl acetate matrix polymeric binder in a consumer latex paint can be retained in the dried paint film permanently. Softening oligomers were incorporated into the paint where the oligomer was retained permanently in the final paint film. The wet paint does not generate an odor or otherwise emit VOC's while drying nor does a residual odor emit from the dried paint film. The softening oligomeric modifiers were found to externally modify the polyvinyl acetate but did not become coreacted with the polyvinyl acetate polymeric binder. The softening oligomeric modifiers were found to function by a chain-spacing mechanism to soften the polyvinyl acetate polymers whereby the oligomeric modifiers provide low temperature film formation and tack-free films less prone to soiling at a given hardness and/or flexibility than ordinarily possible. The discovery enabled the use of essentially all hard polymer units of polyvinyl acetate without the need for internal plasticization (coreaction) with soft butyl acrylate polymeric units.
According to the process of said Ser. No. 019,633 filed Feb. 18, 1993, a compatible organic solution of oligomeric modifier in ethylenic monomer was subjected to high energy shear to advantageously prepare a sub-micron size organic phase dispersed into water. Subsequent polymerization of the micronized monomer droplets produced a softened modified latex very different from conventional emulsion or suspension polymerization polymers. This process provided an excellent micro suspension polymer and was thought to be necessary to accomplish sub-micron aqueous emulsification of the monomer containing the dissolved oligomeric modifier, since the oligomer was found not readily diffuse during polymerization from particle to particle across the aqueous phase.
It now has been found that a low molecular weight oligomer can be surprisingly dispersed by high shear into water under heat and pressure to provide a submicron emulsified aqueous mixture of oligomer stably dispersed in water. The micro-emulsified mixture in turn, can be readily mixed with various emulsion latex polymers to provide a stable mixture of dissimilar emulsions. It was found that micro-emulsions of oligomer in water having an average droplet size less than about one micron enable the oligomer particle to migrate out of the oligomer droplets then through the aqueous phase and into surrounding latex polymer particles if the oligomer is sufficiently low molecular weight oligomer. The lowest molecular weight oligomer particles move faster due to micro small size and have a small but finite solubility in water. A medium range molecular weight fraction of the low molecular weight oligomers move through the water phase into the latex particles at a slower rate extending over several hours or even a few days. The higher molecular weight fraction of the low molecular weight oligomer have essentially zero solubility in water and invariable remain within the oligomer emulsion droplets until a paint film dries, at which time these oligomer particles physically contact the latex particles and eventually migrate into the latex polymer particles during drying.
In accordance with the process of this invention, low molecular weight oligomer having a number average molecular weight below about 5,000 is stably dispersed into water assisted with surfactants by heating a mixture of oligomer and water containing by weight between about 40% and 70% oligomer at temperatures preferably between about 45.degree. C. and 60.degree. C., and then micronizing the heated oligomer water mixture under substantial shear to provide the stable microemulsion of oligomer dispersed aqueous emulsion having an average microemulsion droplet size less than about ten microns. The resulting oligomeric preformed microemulsion can be blended with a wide variety of latex polymers to provide a polymeric binder for paint coatings. Preferred oligomers are non-volatile oligomers having a molecular weight between about 300 and 5,000. Lower molecular weight compounds tend to be volatile and cause excessive plasticizer migration while higher molecular weight polymers lose low temperature film-forming and softening effects.
The principal advantage of this invention is the elimination of the odor and VOC associated with volatile coalescent solvents which are intentionally volatile and intended to migrate out of the dried paint film. An additional advantage pertains to dried paint films exhibiting superior toughness obtained through the use of a hard matrix polymer balanced with the oligomeric modifier to accommodate softening through the external addition of softening modifier while retaining the desired dried film hardness. A further advantage pertains to lower net cost for both interior and exterior paints since high cost soft monomers can be avoided. Volatile coalescing agents can be eliminated and binder volume can be increased by using a permanent non-volatile softening oligomer instead of a volatile coalescent. Other volatile organic solvents can be eliminated enabling a zero VOC coating. The resulting dried paint films exhibit a superior balance of hardness and flexibility while maintaining long term resistance and flexibility. These and other advantages of this invention will become more apparent by referring to the detailed description and illustrative examples.